[phobos] Pureness of enforce()

[Lars Tandle Kyllingstad]

On Tue, 2010-11-09 at 08:48 -0800, Andrei Alexandrescu wrote:
> On 11/9/10 8:34 AM, Don Clugston wrote:
> > On 9 November 2010 16:15, Robert Jacques<sandford at jhu.edu>  wrote:
> >> On Tue, 09 Nov 2010 06:21:38 -0500, Lars Tandle Kyllingstad
> >> <lars at kyllingen.net>  wrote:
> >>>   It is not possible to annotate
> >>> enforce() with 'pure', however, because it takes a lazy parameter, which
> >>> is just shorthand for a (possibly impure) delegate.
> >
> >> Well, long term, we need modifiers to apply to delegates. i.e. it should be
> >> possible to declare a pure delegate.
> >
> > You can already do that.
> >
> > int delegate(int x, int y) pure @safe foo;
> 
> Cool! Then overloading enforce on purity should be the solution.


I am not quite satisfied with the conclusion of this discussion.  There
are currently two big problems with enforce() taking a lazy parameter:

1. It prevents enforce() from being marked as pure, thus preventing
major parts of Phobos from being pure, thus again preventing a lot of
user code from being pure.

2. It prevents any function that uses enforce() from being inlined, thus
degrading performance.  (See the "std.algorithm.sort slow as molasses"
discussion, from July, on this list.)

Solving these two issues require changes to the compiler, and I suspect
it's not at the top of the priority list.

Until that happens, can we please follow Steve's suggestion and make
enforce() non-lazy and temporarily add a lazyEnforce() for those cases
where the error message is actually expensive to construct?

-Lars